Examples of prepreg include a plain fabric of inorganic fibers, such as glass fibers, or organic fibers, such as resin fibers, containing heat-curable resin impregnant, such as epoxy resin impregnant (refer to, for example, Patent Document 1).
Since a fabric has irregularities on the surface thereof, the irregularities easily appear on a produced prepreg described in Patent Document 1. Accordingly, the prepreg described in Patent Document 1 has a problem in smoothness. In particular, since a conductor circuit pattern is formed on a surface of a prepreg used for producing a circuit board, a sufficient smoothness of the surface of the prepreg is required.
In addition, a uni-direction (UD) prepreg including a parallel fiber group having a plurality of parallel fibers arranged therein and containing heat-curable resin impregnant has been developed (refer to, for example, Patent Document 2). A UD prepreg is formed by spreading twist yarn using a fiber spreading apparatus so as to generate a parallel fiber group of the spread fibers and impregnating the parallel fiber group with a heat-curable resin (refer to, for example, Patent Document 3).
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2002-198658
[Patent Document 2] Japanese Examined Patent Application Publication No. 10-508720
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2005-29912